As is well known, the prior art integrating circuit comprises a resistor connected between an input terminal and an output terminal, and a capacitor connected between the output terminal and a grounded terminal, or a plurality of such circuits connected in cascade. Such a circuit provides only a small output in response to a weak input because it consists of passive circuit elements only. While such a circuit has been widely employed especially in a vertical synchronizing signal separation circuit of a television receiver set, it has the disadvantage that a synchronizing pulse that has a sufficiently large magnitude for vertical synchronization cannot be obtained from a composite synchronizing signal if the received electromagnetic wave is of low intensity. In addition, for the purpose of enlarging the time constant of the integrating circuit to sufficiently attenuate a horizontal synchronizing signal at the output, the impedances of the respective elements are large, or two or three stages of such integrating circuits are cascaded. In this case, in addition to an undesired increase of the attenuation of the vertical synchronizing signal at the output the input and output impedances are high, resulting in impedance mismatching with other circuits to be connected to the integrating circuit.
As an approach for overcoming the above-described disadvantages, it has been proposed to connect a pulse amplifier just behind the integrating circuit, and thereby the magnitude of the vertical synchronizing signal obtained at the output in response to a weak input signal will be improved to a certain extent. However, the separation from the horizontal synchronizing pulse has not been improved because the integration is achieved by the integrating circuit that cannot achieve an active operation.